In the field of manufacture of tubular products formed from multi-layer paper construction, the physical strength characteristics of said products based on a particular paper material, which may relate to tensile strength, crush resistance, tear or puncture resistance or other parameters, is determined to a primary extent by the number of layers of paper incorporated in the tube wall, and the properties of the adhesives used to bind the paper layers together. Typically, greater strength is achieved primarily by incorporating more layers of paper in the tube wall. Similarly, greater resistance to loss of strength in unfavourable environments, such as humid or damp environments, is achieved by using stronger or moisture resistant adhesives, such as sodium silicate or PVA adhesives.
Paper tubes and cores are traditionally formed by gluing multiple layers of paper (typically Kraft paper or similar fibrous paper) to each other whilst winding each layer around a steel mandrel of a given diameter. The process usually involves supplying the various layers of these tubes on individual rolls or other dispensers, concurrently drawing the materials from the dispensers in a predetermined array; applying adhesive concurrently to different strands not in contact with one another; and then winding the array into a tube structure around a mandrel. This technique is known as “spiral winding” in the art and will be understood as such in the following description and claims where this term is used. Equipment for performing this process is supplied, for example, by Pack Industrie, of rue Gutenberg-B.P. 109-68170 RIXHEIM (Alsace)-FRANCE.
Typically the number of layers of paper to be used depends upon the physical requirements required for the tube, eg small diameter paper tube for toilet paper cores would require a small number of thin layers of paper using cheaper dextrin adhesives; a core for rolls of paper weighing up to 2 tonnes may require Brown Chip Board or Liner Board with silicate or PVA glues; or a tube for casting concrete into would require greater wall thickness with glues of greater water resistance, therefore requiring a greater number of layers of paper.
However, these measures typically make the tubes heavier, more difficult to transport due to mass and/or lack of flexibility, more difficult to cut open where necessary, and more difficult and expensive to manufacture. In addition, silicate based adhesives are more expensive, and present various safety issues during manufacture. For example, some silicate-based adhesives are likely to set very hard, and residues can tend to present a cut hazard. In addition, some water resistant adhesives take a longer time to cure, sometimes requiring assistance by curing in ovens etc, which may increase the unit cost and capital cost of manufacturing lines.
Accordingly, it is necessary to find ways of improving various of the strength parameters of paper-based tubing which overcomes some or all of these issues with respect to various of the applications to which these tubes are put.
For example, in the building and construction industry, the use of multiple layer paper based tubes and formwork in the casting of vertical concrete piles has become common practice in setting the foundations for buildings, especially larger multiple-storey constructions.
The typical construction used in the prior art for single-use casting tubes is to create a rigid tube from multiple layers of kraft or other fibrous paper. However, as discussed above, one issue encountered with kraft or other fibrous paper construction is that it tends to lose tensile strength when wetted, causing bulging or failure. Also, in humid conditions, the dimensions of the tube can tend to alter, which makes control of the dimensions of the concrete pile difficult.
An approach taken in the prior art to the construction of such tubes is exemplified in U.S. Pat. No. 5,376,316 (Weekers), wherein there is described a tube being made essentially from multiple layers of kraft paper. This particular casting tube also includes a layer of plastic material within the layers of kraft paper for the express purpose of improving the waterproofing properties of the tube. However, this document does not suggest how the overall strength of the tube may be improved. For example, a drawback with this construction is that while the plastic layer may provide some barrier to wetting of the kraft paper layers, it cannot completely prevent weakening of these layers where water might enter from both sides, or especially if water enters through the ends of the tube.
In any case, there are concrete forming applications where a semi-rigid or rigid tube is necessary. The prior art does not teach any method of strengthening and/or waterproofing such tubes. Presumably, the person faced with this problem must resort to the above-discussed methods involving further paper layers and/or expensive adhesives.
Equally, where such paper-based tubes are to be used as packaging material for hard, sharp and/or heavy materials, such as metal components, a major issue is that these items tend to place a great deal of localised impact strain on the tube wall as they shift during transport. Again, the construction disclosed in Weekers would not be sufficiently rigid to reliably be used for this kind of packaging measure, but the only other solution available in the prior art would appear to relate to the well-known route of applying more and more layers of paper to the tube.
Similarly, when attempting to create a tube having superior crush resistance, for example for use as a heavy duty core for rolled products such as newsprint, the prior art teaches only the familiar solution of applying more and more layers of paper to the tube and different types of paper.
Therefore, it is an object of the present invention to provide a multi-layer tubing material of improved physical properties, which does not require excessive paper layering or the use of expensive or hazardous adhesives, and which may be manufactured using conventional paper tube forming equipment.